Range imaging systems are used in a variety of applications to determine the three-dimensional (3-D) characteristics of a scene (a scene is an environment of interest). By way of example, these applications include 3-D scene reconstruction, 3-D object recognition, robot navigation, terrain mapping and object tracking. An important component of a range imaging system is a range camera. A range camera is a device that is used to measure a 3-D structure of a scene by providing range (or depth) information as measured from a plane on the camera. Thus, while a black and white camera provides a grayscale intensity of each pixel and a color camera provides a color of each pixel, a range camera provides a range (or distance to the 3-D scene) of each pixel. Range cameras use a variety of techniques to measure range including lasers, projected light patterns and stereo vision.
For some applications (such as tracking persons within a scene) the range imaging system may include more than one range camera because a single range camera may not have a sufficiently large field of view to monitor the entire scene. In order for multiple range cameras to work together, however, the cameras must be calibrated to determine a position and an orientation of each camera relative to one of the cameras (known as a relative pose). This calibration of multiple cameras enables the ranging system to convert 3-D measurements obtained from each camera into a common coordinate frame. For example, a path of a person in a scene may be measured by each camera in its local coordinate frame and converted to a common coordinate frame (such as a room-based coordinate system).
Several types of manual calibration techniques are used to calibrate the range cameras. One type of calibration technique uses a three-dimensional calibration chart to determine the relative position of each camera. This technique, however, is difficult to use and time-consuming because it requires that the calibration chart be positioned correctly within a scene.
Another type of calibration technique requires a user to monitor a scene and determine a plurality of reference points in the scene until the relative position of each camera can be determined. For example, a user references a number of common points in a scene (within each camera's field of view) and, if enough of these common points are found, the relative pose of the cameras may be determined. One disadvantage of this technique, however, is that it is difficult to implement in a consumer-based product because it is unlikely the consumer would want to perform such a complicated and time-consuming calibration process. Moreover, with both types of calibration techniques, if the consumer performed the calibration process improperly any results obtained from the range imaging system would be erroneous.